An adhesive prepared by blending a water soluble polymer and an isocyanate compound into a water-based polymer dispersion is well known as a water-based polymer-isocyanate adhesive, which provides an extremely high adhesive strength only by a short-time pressing at normal temperature or under warming. Moreover, the adhesive has attracted attention in recent years in relation to indoor environmental issues because it does not generate formaldehyde different from conventional amino or urea resin based adhesives.
This water-based polymer-isocyanate adhesive is excellent not only in dry adhesive performance, but also in secondary performance such as water-resistant adhesion, boiling-water-resistant adhesion, and heat-resistant adhesion, and it exhibits excellent performance for the adhesion of wood. For example, it is widely used for glued lamination boards, plywood and the like. However, conventional water-based polymer-isocyanate adhesives are insufficient in water-resistant adhesion, particularly in boiling-water-resistant adhesion due to the diversification of the types of wood in recent years, although they are satisfactory in dry adhesion. Further, bonded wood may be used in an environment severer than conventional environments, for example, being left under high temperature and high humidity conditions for a long period of time. Under these circumstances, further improvement in the water-resistant adhesion and boiling-water-resistant adhesion of the water-based polymer-isocyanate adhesive is required in the market.
In addition to the requirement for the improvement in adhesion, an extension of stability (pot life) of the adhesive itself after an isocyanate compound is blended is also desired. When the improvement in adhesion is attempted, increase in the. reactivity in curing of an isocyanate compound may be a target, which is, however, not preferred because it leads to decrease in the pot life at the site of bonding. Therefore, these two points as described above, that is, improvement in adhesion and extension of pot life, are greatly desired.
With reference to these requirements, Patent Document 1 discloses an invention of a water-based polymer dispersion for wood adhesion having a core-shell structure. Patent Documents 2 and 3 each disclose an invention of a combination of a water-based polymer dispersion having a core-shell structure and an isocyanate compound.
However, Patent Document 1 does not satisfy the water-resistant adhesion and boiling-water-resistant adhesion that is required at present because a polymer in a first step is a vinyl acetate copolymer. Patent Document 2 does not satisfy the boiling-water-resistant adhesion that is required at present because a vinylester polymer is used as one of the polymers. Patent Document 3 does not satisfy the water-resistant adhesion and boiling-water-resistant adhesion that is required at present because a shell layer has a glass transition temperature of 80° C. or higher. Moreover, there is absolutely no suggestion of the extension of pot life. Thus, the requirement for the pot life that is desired is not satisfied.
Patent Document 1: JP-A-02-302485
Patent Document 2: JP-A-05-78643
Patent Document 3: JP-A-10-195173
It is an object of the present invention to provide a water-based polymer dispersion which is suitable for an adhesive composed of a composition containing a water soluble polymer, a water-based polymer dispersion, and an isocyanate compound and is excellent in water-resistant adhesion, boiling-water-resistant adhesion, and heat-resistant adhesion without sacrificing the pot life of the adhesive.